A ball screw is a mechanical element that includes a ball screw shaft and a ball screw nut. The shaft is formed with a helical ball rolling groove on its outer circumferential surface. The ball screw nut is formed with a helical ball rolling groove on its inner circumferential surface. A number of balls are rollably contained within a ball rolling passage formed by the oppositely arranged helical ball rolling grooves. The ball screw is adapted to convert a rotational motion of the ball screw shaft or the ball screw nut into an axial translational motion of the ball screw nut or the ball screw shaft.
Usually, the ball rolling groove of the ball screw nut is formed by cutting and grinding its inner circumferential surface. Firstly, a prepared bore is formed in a blank by drilling. The helical ball rolling groove is formed on its circumferential surface of the prepared bore by cutting with the use of a turning tool. After performing a heat treatment, such as carburizing hardening, an outer circumference of the blank is ground by a cylindrical grinding machine. Finally, the surface of the ball rolling groove formed by cutting is ground with a grinding stone.
Recently, in ball screw used in automobile actuators, various kinds of ball screws have been proposed to satisfy the demands to reduce the price of the ball screws. As shown in FIG. 7, one example has been proposed for a manufacturing method of a ball screw nut. It includes a step of drilling a predetermined inner circumference (bore) at the center of a blank (step P1). A step to finish outer and inner circumferences of the blank with the use of a turning tool (step P2). A step to insert a tapping tool into the inner circumference and cut the ball rolling groove thereon (step P3). A step to heat treat (carburizing hardening) the ball rolling groove to form a predetermined hardened layer (step P4). A step to shot peen, with steel beads, after the heat treatment (step P5).
Adoption of this prior art manufacturing method eliminates grinding of the ball rolling groove using a conventional grinding wheel. Thus, this reduces the manufacturing cost. In addition, it also eliminates the generation of abnormal layers such as a grain boundary oxidation layer on the surface of the ball rolling groove by using the shot peening process. Also, it improves surface characteristics such as surface hardness and residual compression stress etc. giving influences to the life of the ball screw. (Japanese Laid-open Patent Publication No. 90570/2005.)
However, a problem exists in that regions of the ball rolling groove cannot be directly struck by media, such as steel beads, used in the shot peening process during its performance, as schematically shown in FIG. 8. Thus, it is necessary to eject media again in a reverse direction of ejection. This additional ejecting process not only increases the manufacturing cost but also causes differences in the surface characteristics between regions struck twice by the media and regions struck only once by the media. Thus, a deterioration in accuracy of the ball rolling groove occurs.